An important goal in designing a wireless communication system is to maximize system capacity, that is, to maximize a number of users that may simultaneously be served by the communication system. One way of increasing system capacity is to use a smart antenna system. Smart antenna systems use an array of antenna elements to wirelessly transmit information to a target mobile station (MS). The use of an array of antenna elements allows a transmitted signal to be beamformed so that a narrower, more focused beam is transmitted to the MS. That is, by adjusting the amplitudes and phases of the signals conveyed to each antenna element, a desired beam may be formed. By beamforming the transmitted signal, multipath fading of the transmitted signal and interference with non-targeted users is reduced since the beam is more narrowly focused.
There are two primary types of smart antenna systems, switched beam antenna systems and adaptive antenna systems. In a cellular communication system, a switched beam antenna system comprises a use of multiple predetermined, fixed beams in a sector of a cell. The outputs of the multiple antenna elements of the switched beam antenna system are combined in such a way as to form narrow, directional beams that are spatially selective. As a target MS moves through the sector, the switched beam system switches the information intended for the MS from one beam to another. That is, when the MS is in a first beam of the multiple beams, all of the transmit power available in the sector is allocated to the first beam. When the MS is in a second beam of the multiple beams, all of the transmit power available to the sector is allocated to the second beam.
By confining the signal transmitted to a particular MS to a beam containing the MS, system performance is improved since confinement of the transmitted signal to the correct beam reduces interference observed by other mobile stations not in this beam. In addition, confinement of the signal transmitted to a particular MS to a beam containing the MS permits a reduction in transmit power, and concomitantly in interference observed by the other mobile stations. As a result, system capacity is improved since the reduction in interference permits increased frequency reuse, in a Time Division Multiple Access (TDMA) communication system, or orthogonal code reuse, in a Code Division Multiple Access (CDMA) communication system.
However, utilization of a switched beam antenna system to focus a beamed signal upon a target MS moving through a sector only begins to address the issue of increasing system capacity, which is a continuing constraint upon operation of wireless communication systems. Therefore, a need exists for a method and apparatus for obtaining additional capacity improvements through utilization of a switched beam antenna system in a wireless communication system.